New research from the Massachusetts Institute of Technology reveals
that marine cyanobacteria, whose body mass forms the base of the ocean food chain, also feed
marine organisms in another way - they deliver "food parcels" packed with carbon and other
nutrients.

The bacteria release lipid vesicles - spherical sacs containing proteins and genetic
material in the form of DNA - and RNA, which the researchers suggest provide a means of gene
transfer in bacterial communities and could also act as decoys for viruses.

Marine cyanobacteria are like tiny ocean plants in that they make their own carbon-rich food
from photosynthesis, which uses sunlight and CO2. Thus, through their own body mass, they provide
the ocean's food chain with organic compounds and also with oxygen as a byproduct of
photosynthesis.

In a recent issue of Science, Sallie W. Chisholm, a professor in the Massachusetts Institute of Technology's (MIT's) department
of civil and environmental engineering, and colleagues report how they discovered large
numbers of extracellular vesicles - each measuring about 100 nanometers across - linked to the
two most abundant types of cyanobacteria, Prochlorococcus and
Synechoccocus.

First time extracellular vesicles linked to ocean bacteria

The knowledge that bacteria release extracellular vesicles has been around since the
1960s, but this is the first time it has been observed in ocean bacteria.

The team found the vesicles in cultures of cyanobacteria and also in samples taken from the
nutrient-rich waters around the coast of New England, as well as the nutrient-sparse waters of the
Sargasso Sea, in the middle of the North Atlantic Gyre.

When they tested them in the lab, they found the vesicles to be stable and able to last 2
weeks or more, offering enough carbon to sustain the growth of bacteria that do not use
photosynthesis.

Discovery will change the way we think about ocean's food cycle

Finding these vesicles are so abundant in the oceans means we have to change the way we
think about them and their role in the ocean's food cycle - a key message from the study.

We know little about how they contribute to the circulation and supply of dissolved organic
carbon in marine ecosystems. They could be an important way that organisms in the sea exchange
genes and other essential materials, energy and information.

When they analyzed the genetic material in the vesicles taken from the seawater, the team
found DNA from a wide range of bacteria, suggesting many of them produce vesicles.

Just Prochlorococcus's global production amounts to some billion billion billion
vesicles per day - contributing a significant amount of carbon-rich material to the sparse
nutrient pool of the open seas, they note.

What is the evolutionary advantage of giving away food in vesicles?

But why is a bacterial cell prepared to release a packet one-sixth of its own size every
day, especially in the nutrient-sparse environment of the open seas? The researchers wondered why they would take such a risk.

Prof. Chisolm says:

"Prochlorococcus is the smallest genome that can make organic carbon from sunlight
and carbon dioxide and it's packaging this carbon and releasing it into the seawater around it.
There must be an evolutionary advantage to doing this. Our challenge is to figure out what it
is."

One explanation might lie in the fact Prochlorococcus relies on non-photosynthetic
bacteria to break down chemicals that are toxic to it - it has lost the ability to do it for
itself.

So perhaps, by sending tasty little snack parcels to its non-photosynthetic neighbours,
Prochlorococcus is keeping the relationship mutually beneficial.

Another idea the researchers suggest is that the vesicles act as a decoy for predators.
Under electron microscopes they could see how phages - viruses that attack bacteria - became
attached to vesicles.

Once a phage injects its DNA into a vesicle, it is effectively disarmed and rendered
ineffective - it cannot then reproduce itself in a living cell. It is as though the bacteria
release the vesicles in a similar way to fighter jets that release chaff to divert missile
attacks.

The MIT Energy Initiative, along with grants from the Gordon and Betty Moore Foundation and the
National Science Foundation's Center for Microbial Oceanography, helped finance the study.

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